The stalled investigation into a gruesome 2011 triple murder in the leafy suburb of Waltham, Mass., has taken a dramatic turn in the days since two explosions ripped through the crowd during the final stretch of the Boston Marathon.

In the two weeks since those blasts erupted 12 seconds apart, FBI agents have begun working with prosecutors to interview family members and friends of the three victims in the unsolved Waltham slayings, law enforcement authorities tell ABC News. Family members of two victims told ABC News they had been contacted by FBI agents about the case in recent days. The agents' interest has been sparked by an overlooked connection between accused bomber Tamerlan Tsarnaev and one of the victims in the case, Brendan Mess.

Mess and his two friends; Raphael Teken, 37, and Erik Weissman, 31, had their throats slit late on Sept. 11, 2011, the 10-year anniversary of the 9-11 attacks. Last week, prosecutors confirmed to ABC News that they were trying to determine whether Tsarnaev had any link to the case.

Both Tsarnaev, 26, a Golden Gloves champion, and Mess, 25, a Jiu-Jitsu fighter, had harbored dreams of a career in mixed-martial arts, a sport which combines skills from both. The two started training together in 2009, and at one point lived as roommates, relatives told investigators. In the months before the slayings, friends of Mess said he had been bringing the quiet immigrant along with him to social events.

Scott Wood, 40, an army ranger and instructor at Vermont Brazilian Jiu-Jitsu, befriended and trained Mess when he was briefly a student at the University of Vermont. He says the last time he saw Mess was at a fight event in June 2011. Mess brought Tsarnaev as his guest.

"I met that guy at a fight event. Brendan introduced me to him as Tam,'' Wood told ABC News. "He seemed like a real arrogant guy, the tough guy at the gym, not a mass murderer."

Another friend of Mess's told ABC News he, too, remembered Tsarnaev joining the group to watch a mixed-martial arts fight. The friend, who spoke on the condition he not be identified, said Mess and his friends kept calling Tsarnaev "champ," because of his Golden Gloves title. Tsarnaev was quiet, he said, sitting at the end of the row and declining when people offered him beers.